To remove dust-laden air from an ambient atmosphere where dust and particles are generated and to capture and collect the airborne dust and particles, various types of dust collectors have been used which may be grouped into wet-type dust collectors and dry-type dust collectors.
Wet-type dust collectors known in the art include a cyclone scrubber, a leaking shelf washing tower, a venturi scrubber, a packed tower, an impingement scrubber, a water film scrubber, etc.
Generally, these wet-type dust collectors are large in scale, complex in structure, and expensive to build, and in many cases are unable or difficult to transport. Moreover, the threshold particle size of the wet-type dust collectors is generally in the order of micrometers so that they are not suitable to collect more fine particles.
Dry-type dust collectors typically are of the filtration type provided with filters and are adapted to draw dust-laden air by a source of vacuum such as a blower, vacuum pump and ejector and to cause the dust-laden air to pass through the filters to remove the dust and particles by filtration.
In the dust collectors of the filtration type, filters are clogged by dust during use so that it is necessary to cyclically backwash the filters by injecting compressed air. Depending on the nature of the dust and particles, e.g., in the case that the particles are fine or hygroscopic, the air flow resistance across the filters will be undesirably increased even though backwashed very frequently.
Therefore, the present inventor has previously proposed a hybrid-type dust collector wherein a wet-type dust collector structure consisting of a bubbling-type cyclone scrubber is arranged upstream of a filtration stage comprised of filters to ensure that a part of dust and particles in the air to be treated is preliminarily collected by the cyclone scrubber to thereby reduce the load upon the subsequent filtration stage (JP-C-3,351,735).
The hybrid-type dust collector mentioned has an upright tubular main body closed at both ends, a tubular partition wall of a reduced diameter being concentrically arranged internally of the main body to define a centrally located filtration chamber and an annular cyclone chamber situated outwardly of the filtration chamber. One or more filters are arranged in the filtration chamber and means is provided for applying suction or vacuum to the secondary side of the filters. The main body is provided with an air inlet opening tangentially to the upper part of the cyclone chamber. The lower end of the partition wall is spaced upwardly away from the bottom of the main body so that the filtration chamber and the cyclone chamber are communicated with each other at the lower part thereof. In use, water is introduced onto the bottom of the main body to form a water sump in which the lower end of the partition wall is submerged.
The hybrid-type dust collector just described is designed to particularly process dust-laden moist air containing water vapor in such a manner that, as the dust-laden air is drawn into the cyclone chamber and water droplets are formed by condensation following expansion and cooling of water vapor, the dust and powders in the dust-laden air are captured and collected in the primary step by impinging upon the inner wall of the cyclone chamber together with the water droplets. The dust and powders are further collected in the secondary step by the air-to-liquid contact carried out during bubbling as air is caused to submerge into the water sump as it passes the lower end of the partition wall. The dust and particles are further caught in the final step by the filters.
The dust collector mentioned enjoys the advantage that it is small, compact, simple in structure, and cost effective, because the filtration chamber is located at the center with the cyclone chamber arranged outwardly thereof, and that maintenance thereof is easy because it is washable. Furthermore, the dust collector has an excellent explosion-proof capability because the bubbling-type scrub arrangement acts as a fire wall.
However, under a certain operating conditions, the hybrid-type dust collector involves two problems.
Firstly, since the bubbling-type scrubber is arranged upstream of the filters, fine water droplets generated as air bubbles are burst will be carried by air and will be drawn toward the filters so that the filters will be wetted with water.
When the surface of the filters are wetted with water, the powders and particles collected thereby will, in turn, be wetted with water. As a result, dust and particles thus collected will adhere or stick to the filters so that they will not be dislodged therefrom by backwashing with compressed air. Consequently, filters are clogged with dust causing the air-flow resistance and pressure drop to increase. This will necessitate to increase the output power of the source of vacuum such as a blower of a vacuum pump whereby the running costs and production costs of the system will be augmented.
In particular, in the case where filters for ultra-fine particles capable of collecting particles in the order of submicrons are used, an increase in the air-flow resistance and pressure drop due to wetting will be so high that the running costs and production costs of the system will be prohibitively increased.
Filter wetting due to bubbling will become more dominant as the flow rate and air speed of the dust collector is increased.
Secondly, in the case that airborne dust and particles in the air to be treated are fine and combustible, some explosion-proof measures must be taken in general because the dust-laden air becomes explosive and there is a danger of dust explosion. It is believed that dust explosion is resulted when a minute explosion triggered, for example, by electrostatic spark discharge induces chain explosion of dust deposit. Therefor, formation of dust deposit is most undesirable for dust explosion. In particular, ignition of fine dust and particles may occur even by a smallest energy of spark discharge of electrostatic charges. Furthermore, dust explosion is likely to occur when the air to be treated is dry.
The hybrid-type dust collector described hereinbefore is designed to remove the dust and particles in such a manner that, when the dust-laden air containing water vapor is drawn into the cyclone chamber, water vapor is condensed into water as it is subjected to expansion and cooling, so that the dust and particles, together with the droplets of condensate, are caused to adhere to the inner wall of the cyclone chamber.
Accordingly, this dust collector is not suitable for use in the treatment of explosive, dry dust-laden air. This is because there is a likelihood of the collected dust and particles to deposit onto the inner wall of the cyclone chamber and the air entered through an inlet into the cyclone chamber is directly brought into contact with the dust deposit.
Therefore, when intended for use in the treatment of explosive, dry, dust-laden air, there is a room for the hybrid-type dust collector mentioned to be improved from the view point of explosion-proof capability.
Accordingly, an object of the present invention is to improve the hybrid-type dust collector described in the afore-mentioned patent and to prevent the filter from being wetted with water to thereby prevent an increase in the air-flow resistance and pressure drop across the filter.
Another object of the invention is to improve the afore-mentioned hybrid-type dust collector in a manner to enhance the explosion-proof capability.
A still another object of the invention is to provide a hybrid-type dust collector which is small and compact, cost effective and simple in structure, and easy to maintain, while achieving the foregoing objects.